Organic light emitting element

ABSTRACT

Provided is an organic light emitting element having stable performance in the air. The organic light emitting element includes: an anode; a cathode; and an emission layer placed between the anode and the cathode, in which: the organic light emitting element further includes a first organic compound layer placed between the cathode and the emission layer, and a second organic compound layer placed between the emission layer and the first organic compound layer, and contacted with the first organic compound layer; the first organic compound layer contains a first organic compound; the second organic compound layer contains a second organic compound; and the first organic compound includes a viologen compound represented by the following general formula [1] and the second organic compound includes an organic compound different from the viologen compound.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic light emitting element, a display apparatus, an image information processing apparatus, a lighting apparatus, an image forming apparatus, and an exposing apparatus.

2. Description of the Related Art

An organic light emitting element (organic electroluminescence element or organic EL element) is an electronic element including a pair of electrodes and an organic compound layer placed between the pair of electrodes. An electron and a hole are injected from the pair of electrodes, and then the electron and the hole recombine in the organic compound layer to produce an exciton of a luminous organic compound. The organic light emitting element emits light upon return of the exciton to its ground state.

In the organic light emitting element, at least one organic compound layer is placed between the electrodes. As one layer constituting the organic compound layer, there is known an electron injection layer that serves to inject or transport an electron injected from a cathode into an emission layer. For example, an alkali metal or alkaline earth metal derivative has been widely known as a constituent material for the electron injection layer. It is because each of the alkali metal and alkaline earth metal derivatives is a material having a small work function and shows a good electron injection property that any such derivative is used as the constituent material for the electron injection layer. However, it has been known that each of the alkali metal and alkaline earth metal derivatives is a material that easily reacts with water. Accordingly, an organic light emitting element containing any one of the alkali metal and alkaline earth metal derivatives as a constituent material for its electron injection layer is affected by moisture in the air. Therefore, at present, the organic light emitting element needs to be stringently sealed so that the organic light emitting element may not be exposed to the moisture in the air. Meanwhile, in order that the effect due to the moisture in the air may be overcome, various researches and developments have been conducted on a method of stably driving the organic light emitting element even in the air except the sealing.

As one method of improving the stability of the organic light emitting element against the moisture in the air, for example, there is known a method involving introducing Compound a-1 having an electron donor property and Compound a-2 having an electron acceptor property shown below into the electron injection layer like U.S. Patent Application Publication No. 2005/0110005.

In the electron injection layer of the organic light emitting element of U.S. Patent Application Publication NO. 2005/0110005, the compound having an electron donor property (D molecule) donates an electron to the compound having an electron acceptor property (A molecule), whereby charge (an electron) is generated. In addition, a strong interaction occurs between the D molecule and the A molecule to produce a polarized DA complex. Thus, the organic light emitting element of U.S. Patent Application Publication NO. 2005/0110005 can perform the injection of an electron.

J. Org. Chem. 2008, 73, 445-450 discloses Viologen Compound b-1 shown below as a compound having a high electron donor property.

However, the HOMO of Compound a-1 (compound having an electron donor property) is deep and hence the energy level of the DA complex to be formed in the organic light emitting element of U.S. Patent Application Publication No. 2005/0110005 is deep. Therefore, when Compounds a-1 and a-2 are used as constituent materials for the electron injection layer constituting the organic light emitting element, an electron can be accepted from an electrode (cathode) but a problem occurs in that a barrier for the injection of an electron into an emission layer is large and hence good light emission is not obtained. Meanwhile, Viologen Compound b-1 disclosed in J. Org. Chem. 2008, 73, 445-450 has a high electron donor property but its availability as a constituent material for an organic light emitting element has not been suggested. In addition, Viologen Compound b-1 itself has a large molecular weight and hence it has been difficult to subject the compound to sublimation purification.

SUMMARY OF THE INVENTION

The present invention has been accomplished to solve the above-mentioned problems and the present invention is directed to providing an organic light emitting element having stable performance in the air.

An organic light emitting element according to a first aspect of the present invention includes: an anode; a cathode; and an emission layer placed between the anode and the cathode, in which: the organic light emitting element further includes a first organic compound layer placed between the cathode and the emission layer, and a second organic compound layer placed between the emission layer and the first organic compound layer, and contacted with the first organic compound layer; the first organic compound layer contains a first organic compound; the second organic compound layer contains a second organic compound; and the first organic compound includes a viologen compound represented by the following general formula [1] and the second organic compound includes an organic compound different from the viologen compound.

In the formula [1], R₁ to R₆ each represent a hydrogen atom or a substituent selected from the group consisting of a fluorine atom, an alkyl group, an alkoxy group, an aryl group, and a heteroaryl group, when any one of R₁ to R₆ represents an alkyl group or an alkoxy group, the alkyl group or the alkoxy group may further have a fluorine atom, and when any one of R₁ to R₆ represents an aryl group or a heteroaryl group, the aryl group or the heteroaryl group may further have a fluorine atom, an alkyl group, an alkoxy group, or an amino group.

An organic light emitting element according to a second aspect of the present invention includes: an anode; a cathode; and an emission layer placed between the anode and the cathode, in which: the organic light emitting element further includes an organic compound layer placed between the cathode and the emission layer; the organic compound layer contains a first organic compound and a second organic compound; and the first organic compound includes a viologen compound represented by the following general formula [1] and the second organic compound includes an organic compound different from the viologen compound.

In the formula [1], R₁ to R₆ each represent a hydrogen atom or a substituent selected from the group consisting of a fluorine atom, an alkyl group, an alkoxy group, an aryl group, and a heteroaryl group, when any one of R₁ to R₆ represents an alkyl group or an alkoxy group, the alkyl group or the alkoxy group may further have a fluorine atom, and when any one of R₁ to R₆ represents an aryl group or a heteroaryl group, the aryl group or the heteroaryl group may further have a fluorine atom, an alkyl group, an alkoxy group, or an amino group.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view illustrating an example of a display apparatus including an organic light emitting element of the present invention and an active element connected to the organic light emitting element.

FIG. 2 is a schematic view illustrating an example of an image forming apparatus including the organic light emitting element according to the present invention.

FIGS. 3A and 3B are each a schematic plan view illustrating a specific example of an exposure light source constituting the image forming apparatus of FIG. 2.

FIG. 3C is a schematic view illustrating a specific example of a photosensitive member constituting the image forming apparatus of FIG. 2.

FIG. 4 is a schematic view illustrating an example of a lighting apparatus having the organic light emitting element according to the present invention.

DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings.

The present invention relates to an organic light emitting element including an anode, a cathode, and an emission layer placed between the anode and the cathode. The organic light emitting element of the present invention including a first organic compound layer and second organic compound layer described in the following (A), or an organic compound layer described in the following (B).

(A) A first organic compound layer placed between the cathode and the emission layer, and a second organic compound layer placed between the emission layer and the first organic compound layer, and contacted with the first organic compound layer (B) An organic compound layer placed between the cathode and the emission layer

When the organic light emitting element of the present invention includes the two layers (the first organic compound layer and the second organic compound layer) described in the (A), the first organic compound layer contains a first organic compound and the second organic compound layer contains a second organic compound. In the present invention, the first organic compound is a viologen compound represented by the following general formula [1]. Meanwhile, in the present invention, the second organic compound is an organic compound different from the viologen compound represented by the following general formula [1].

It is to be noted that details about the viologen compound represented by the general formula [1] and the second organic compound are described later. In addition, when the organic light emitting element of the present invention includes the two layers (the first organic compound layer and the second organic compound layer) described in the (A), the first organic compound layer is preferably a layer contacted with the cathode.

Further, when the organic light emitting element of the present invention includes the organic compound layer described in the (B), the organic compound layer contains a first organic compound and a second organic compound. In the present invention, the first organic compound is a viologen compound represented by the general formula [1]. Meanwhile, the second organic compound is an organic compound different from the viologen compound represented by the general formula [1]. When the organic light emitting element of the present invention includes the organic compound layer described in the (B) as just described, the organic compound layer is preferably a layer contacted with the cathode.

[Organic Light Emitting Element]

Hereinafter, a specific construction of the organic light emitting element of the present invention is described.

In the organic light emitting element of the present invention, at least the emission layer is placed between the anode and the cathode as a pair of electrodes. In the present invention, the emission layer and an electron injection layer as a layer closest to the cathode are preferably placed between the anode and the cathode. However, in the present invention, layers to be placed between the anode and the cathode are not limited to the emission layer and the electron injection layer. In addition to the emission layer and the electron injection layer, a layer such as a hole injection layer, a hole transport layer, a hole blocking layer, an electron transport layer, or an exciton blocking layer may be appropriately introduced.

In addition, in the present invention, for example, an insulating layer, an adhesion layer, or an interference layer may be formed at an interface between an electrode and a layer formed between the electrodes (such as the emission layer). Further, when a charge transport layer (an electron transport layer or a hole transport layer) is incorporated as a layer to be formed between the electrodes, the charge transport layer may be constituted of a plurality of layers having different ionization potentials. As described above, the organic light emitting element of the present invention can adopt various layer constructions.

In the present invention, the light extraction construction of the organic light emitting element may be a top emission system in which light is extracted from an electrode on a substrate side, or may be a bottom emission system in which light is extracted from a side opposite to the substrate. A construction in which light is extracted from each of both the side closer to the substrate and the sides is also permitted.

[Viologen Compound (First Organic Compound, Electron Donor-Property Compound X)]

Next, the viologen compound represented by the following general formula [1] (first organic compound) as one constituent material for the organic light emitting element of the present invention is described. It is to be noted that the viologen compound represented by the general formula [1] is a compound having an electron donor property, and is preferably used as a constituent material for a layer between the cathode and the emission layer. The compound is more preferably used as a constituent material for a layer contacted with the cathode and is particularly preferably used as a constituent material for the electron injection layer. In addition, in the following description, the viologen compound represented by the general formula [1] is sometimes referred to as “electron donor-property compound X.”

In the general formula [1], R₁ to R₆ each represent a hydrogen atom or a substituent selected from the group consisting of a fluorine atom, an alkyl group, an alkoxy group, an aryl group, and a heteroaryl group.

Examples of the alkyl group represented by any one of R₁ to R₆ include a methyl group, an ethyl group, a n-propyl group, an isopropyl group, a n-butyl group, a tert-butyl group, a n-hexyl group, a n-heptyl group, and a n-octyl group.

The alkyl group represented by any one of R₁ to R₆ is preferably an alkyl group having 1 to 12 carbon atoms. This is because of the following reason: as the number of carbon atoms of the alkyl group increases, the molecular weight of the entirety of the compound increases by an amount corresponding to the increase, which makes it difficult to subject the compound to sublimation purification. However, the alkyl group has an effect of forming a good amorphous film. In addition, the alkyl group is a substituent having an electron-donating effect and hence the introduction of the group into the electron donor-property compound X represented by the general formula [1] can additionally reduce the oxidation potential of the compound itself.

An example of the alkoxy group represented by any one of R₁ to R₆ is a methoxy group.

The alkoxy group represented by any one of R₁ to R₆ is preferably an alkoxy group having 1 to 12 carbon atoms. This is because of the same reason as that of the alkyl group. In addition, the alkoxy group is a substituent having an electron-donating effect larger than that of the alkyl group and hence the introduction of the group into the electron donor-property compound represented by the general formula [1] can additionally reduce the oxidation potential of the compound itself.

It is to be noted that when any one of R₁ to R₆ represents an alkyl group or an alkoxy group, the alkyl group or the alkoxy group may further have a fluorine atom. That is, at least part of hydrogen atoms in the alkyl group or the alkoxy group may be substituted with a fluorine atom. The alkyl group or alkoxy group as a substituent at least part of the hydrogen atoms of which are substituted with fluorine as described above exhibits the following effect: by virtue of the hydrophobic effect or oleophobic effect of a fluorine atom, the compound itself has additional difficulty in reacting with moisture or oxygen in the air and is improved in sublimation property.

Examples of the aryl group represented by any one of R₁ to R₆ include a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthryl group, and a fluorenyl group.

The aryl group represented by any one of R₁ to R₆ is preferably an aryl group having 6 to 18 carbon atoms. This is because as the molecular weight increases, it becomes more difficult to perform sublimation purification. Examples of the aryl group having 6 to 18 carbon atoms include a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, a phenanthryl group, and a fluorenyl group.

Examples of the heteroaryl group represented by any one of R₁ to R₆ include a pyridyl group, a biphenylpyridyl group, a pyrimidyl group, a triazinyl group, a thienyl group, a furyl group, a pyrrolyl group, an imidazolyl group, a benzimidazolyl group, a triazolyl group, a benzothienyl group, a dibenzothienyl group, a quinolyl group, and a substituent shown below.

In addition, when any one of R₁ to R₆ represents an aryl group or a heteroaryl group, the aryl group or the heteroaryl group may further have a fluorine atom, an alkyl group such as a methyl group, an ethyl group, an isopropyl group, or a tert-butyl group, an alkoxy group such as a methoxy group, or an amino group.

By the way, when a substituent represented by any one of R₁ to R₆ of the viologen compound represented by the general formula [1] is appropriately changed, any one of the physical properties of the compound itself, specifically its oxidation potential, film property, heat stability, and sublimation property can be finely adjusted. Here, while the introduction of an electron-donating substituent reduces the oxidation potential, the introduction of an electron-withdrawing substituent increases the oxidation potential. It is to be noted that out of the substituents (R₁ to R₆) in the formula [1], R₁ and R₂ largely contribute to a change in the oxidation potential.

In the present invention, the viologen compound represented by the general formula [1] (electron donor-property compound X) can be used alone as a constituent material for the electron injection layer of the organic light emitting element. This is because the stack of an extremely thin layer of the compound on an electrode (a metal electrode or the cathode) reduces the work function of the electrode. However, in the present invention, in addition to the electron donor-property compound X, a compound different from the electron donor-property compound X may be introduced into the electron injection layer. In addition, in the present invention, a layer containing the compound different from the electron donor-property compound X may be formed so as to be brought into contact with the electron injection layer. The compound different from the electron donor-property compound X as used herein is preferably an electron acceptor-property compound Y to be described below.

[Electron Acceptor-property Compound Y (Second Organic Compound)]

Next, the second organic compound as one constituent material for the organic light emitting element of the present invention is described. In the present invention, the second organic compound is preferably the electron acceptor-property compound Y as a constituent material for the electron injection layer. In the present invention, the electron acceptor-property compound Y is a compound having the following function: the compound accepts an electron from the electron donor-property compound X and emits the accepted electron toward any other organic compound layer such as the emission layer. In the present invention, the electron acceptor-property compound Y is preferably represented by any one of the following general formulae [2-1] to [2-21], [3-1], and [3-2].

In the formulae [2-1] to [2-21], [3-1], and [3-2], R₂₁ to R₉₉ and R₁₀₃ to R₁₅₉ each represent a hydrogen atom or a substituent selected from the group consisting of an alkyl group, an alkoxy group, an aryl group, a heteroaryl group, and a fluorine atom.

Examples of the alkyl group represented by any one of R₂₁ to R₉₉ and R₁₀₃ to R₁₅₉ include, but of course not limited to, a methyl group, an ethyl group, a n-propyl group, an isopropyl group, a n-butyl group, a tert-butyl group, a n-hexyl group, a n-heptyl group, and a n-octyl group. The alkyl group represented by any one of R₂₁ to R₉₉ and R₁₀₃ to R₁₅₉ is preferably an alkyl group having 1 to 12 carbon atoms.

An example of the alkoxy group represented by any one of R₂₁ to R₉₉ and R₁₀₃ to R₁₅₉ is a methoxy group.

Examples of the aryl group represented by any one of R₂₁ to R₉₉ and R₁₀₃ to R₁₅₉ include, but of course not limited to, a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, an anthracenyl group, a fluorenyl group, a phenanthryl group, a benzophenanthryl group, a chrysenyl group, a fluoranthenyl group, and a benzofluoranthenyl group.

Examples of the heteroaryl group represented by any one of R₂₁ to R₉₉ and R₁₀₃ to R₁₅₉ include, but of course not limited to, a pyridinyl group, a pyrimidinyl group, a triazinyl group, a thienyl group, a furyl group, a pyrrolyl group, an imidazolyl group, and a triazolyl group.

It is to be noted that when any one of R₂₁ to R₉₉ and R₁₀₃ to R₁₅₉ represents an aryl group or a heteroaryl group, the aryl group or the heteroaryl group may further have an alkyl group such as a methyl group, an ethyl group, an isopropyl group, or a tert-butyl group, an alkoxy group such as a methoxy group, an aryl group such as a phenyl group, a heteroaryl group such as a pyridyl group, or a fluorine atom.

In the formula [3-1], a unit Q represents a basic structure represented by any one of the formulae [2-1] to [2-21] or a partial structure including an aromatic ring having 6 to 30 carbon atoms. The partial structure including an aromatic ring having 6 to 30 carbon atoms corresponding to the unit Q is, for example, a partial structure defined by any one of the following (A) to (C). However, in the present invention, the partial structure is not limited to the following.

(A) A complex substituent obtained by combining a basic structure represented by any one of the formulae [2-1] to [2-21] and an aryl group (B) A complex substituent obtained by combining a plurality of aryl groups (C) A condensed polycyclic substituent in which 6 or more carbon atoms each having an sp² hybrid orbital are present (A complex substituent formed of a plurality of partial structures is also included.)

Specific examples of the partial structure corresponding to the unit Q include the following partial structures. However, in the present invention, the partial structure is not limited to the following. It is to be noted that in the formulae, * represents an atomic bonding with a unit P to be described later.

In the formula [3-1], m represents an integer of from 0 to 6.

In the formula [3-1], the unit P represents any one of the following substituents.

(In the formulae, R₁₀₀ and R₁₀₁ each represent a substituent selected from the group consisting of a hydrogen atom, an alkyl group, an alkoxy group, an aryl group, a heteroaryl group, and a fluorine atom. It is to be noted that when any one of R₁₀₀ and R₁₀₁ represents an aryl group or a heteroaryl group, the aryl group or the heteroaryl group may further have an alkyl group, an alkoxy group, an aryl group, a heteroaryl group, or a fluorine atom. * represents an atomic bonding with the unit Q. It is to be noted that specific examples of R₁₀₀ and R₁₀₁ are the same as those of R₂₁ to R₉₉ and R₁₀₃ to R₁₅₉.)

In the formula [3-2], a unit T represents a partial structure including an aromatic ring having 6 to 30 carbon atoms or a partial structure including a five- or six-membered heterocyclic structure formed of a carbon atom and an oxygen atom.

The partial structure including an aromatic ring having 6 to 30 carbon atoms corresponding to the unit T is, for example, a partial structure defined by the following (A) or (B). However, in the present invention, the partial structure is not limited to the following.

(A) A complex substituent obtained by combining a plurality of aryl groups (B) A condensed polycyclic substituent in which 6 or more carbon atoms each having an sp² hybrid orbital are present (A complex substituent formed of a plurality of partial structures is also included.)

Specific examples of the partial structure corresponding to the unit T include the following partial structures. However, in the present invention, the partial structure is not limited to the following. It is to be noted that in the formulae, * represents an atomic bonding with a unit Z to be described later.

In the formula [3-2], n represents an integer of from 0 to 6.

In the formula [3-2], the unit Z represents any one of the following substituents.

(In the formulae, R₁₀₂ represents a substituent selected from the group consisting of a hydrogen atom, an alkyl group, an alkoxy group, an aryl group, a heteroaryl group, and a fluorine atom. It is to be noted that when R₁₀₂ represents an aryl group or a heteroaryl group, the aryl group or the heteroaryl group may further have an alkyl group, an alkoxy group, an aryl group, a heteroaryl group, or a fluorine atom. * represents an atomic bonding with the unit T. It is to be noted that specific examples of R₁₀₂ are the same as those of R₂₁ to R₉₉ and R₁₀₃ to R₁₅₉.)

A compound represented by any one of the general formulae [2-1] to [2-21] is an electron-deficient condensed ring compound. The electron-deficient condensed ring compound preferably functions as an electron acceptor because a n-electron of a condensed ring serving as a basic skeleton is deficient with respect to a benzene ring. Examples of the electron-deficient condensed ring compound include a pyridine compound (formula [2-1]), a pyrimidine compound (formula [2-2]), a triazine compound (formula [2-3]), a quinoline compound (formula [2-4]), a naphthyridine compound (formula [2-6]), a phenanthroline compound (formula [2-7]), a fluoranthene compound (formula [2-8]), oxazole compounds (formulae [2-10] and [2-17]), triazole compounds (formulae [2-11] and [2-18]), imidazole compounds (formulae [2-12] and [2-19]), an oxadiazole compound (formula [2-13]), a thiadiazole compound (formula [2-14]), and a triazole compound (formula [2-15]). However, the electron-deficient condensed ring compound is not limited to those compounds and a compound formed by condensing a benzene ring or the like to any one of those compounds (the formula [2-5], [2-9], [2-16], [2-20], or [2-21]) can also be used as the electron-deficient condensed ring compound. Of course, the compound is not limited to the foregoing and a compound to be used in an electron transport layer is also permitted.

Each of the compounds represented by the general formulae [3-1] and [3-2] is a compound having an electron-withdrawing substituent. Here, the electron-withdrawing substituent increases the dipole moment of the entire molecule of the compound because the electron-withdrawing force of the substituent itself causes polarization in the molecule. Accordingly, the compound preferably functions as an electron acceptor. Examples of the electron-withdrawing substituent include, but not limited to, a substituent corresponding to the unit P (a nitro group, a cyano group, a fluorine atom, a boron fluoride group, a phosphonyl group, or a monovalent fluorinated alkyl group), and a substituent corresponding to the unit Z (such as a carbonyl group, a sulfonyl group, an anhydride group, an imide group, a boron fluoride group, or a di- or higher valent fluorinated alkyl group).

In the present invention, when an organic compound layer containing the viologen compound (electron donor-property compound) and the second organic compound (electron acceptor-property compound) is formed between the cathode and the emission layer, a mixing ratio between both the compounds in the organic compound layer is preferably adjusted in an appropriate manner. Specifically, the content of the second organic compound (electron acceptor-property compound) in the organic compound layer is preferably more than 0 wt % and 80 wt % or less with respect to the total of the viologen compound (electron donor-property compound) and the second organic compound.

[Relationship Between Electron Donor-Property Compound and Electron Acceptor-Property Compound]

In the present invention, the effect of the present invention is exhibited by incorporating the electron donor-property compound and the electron acceptor-property compound into the electron injection layer constituting the organic light emitting element. It is to be noted that the electron injection layer may be of a stacked structure formed of the electron donor-property compound X and the electron acceptor-property compound Y, or may be a mixed layer formed of the electron donor-property compound X and the electron acceptor-property compound Y. In addition, in the present invention, the electron donor-property compound X and the electron acceptor-property compound Y preferably satisfy the formula [4].

|V _(red) −V _(ox)|1.0 V  [4]

In the formula [4], V_(red) represents the first reduction potential value of the electron acceptor-property compound Y and V_(ox) represents the first oxidation potential value of the electron donor-property compound X. It is to be noted that the V_(red) and the V_(ox) are oxidation-reduction potentials obtained by cyclic voltammetry (CV) under the same measurement conditions. Here, the term “the same measurement conditions” means that a solvent, an electrolyte, a working electrode, a reference electrode, a counter electrode, a temperature, and a concentration are kept unchanged. In addition, in the measurement and evaluation of the oxidation-reduction potentials, the half-wave potential value (E_(1/2)) of an oxidation-reduction wave that has been generally used is used.

Further, in the present invention, the electron donor-property compound X and the electron acceptor-property compound Y preferably satisfy the formula [5].

|V _(red) −V _(ox)|≦0.5 V  [5]

The electron injection layer constituting the organic light emitting element of the present invention contains the electron donor-property compound X (viologen compound) and the electron acceptor-property compound Y, and hence the following significant effect is obtained: the element is stably driven even in the air.

By the way, the electron injection layer constituting the organic light emitting element is required to have at least characteristics (i) and (iii) out of the three characteristics listed below.

(i) An ability to accept an electron from the cathode (or an ability to donate a hole to the cathode) (ii) An ability to transport charge (an electron) (in the layer) (iii) An ability to inject an electron into an organic compound layer positioned on a side opposite to the cathode (such as an electron transport layer, a hole blocking layer, or the emission layer)

Unless the layer has at least the characteristics (i) and (iii) out of the characteristics (i) to (iii), an electron injected from the cathode cannot be injected into the emission layer and hence good light emission from the emission layer is not obtained. It is to be noted that an electron injection layer using an alkali metal or an alkaline earth metal, or a derivative thereof as its constituent material has all the characteristics (i) to (iii) but is not preferred because the electron injection layer easily reacts with a component in the air, especially water to result in its alteration.

The organic light emitting element disclosed in U.S. Patent Application Publication No. 2005/0110005 also contains an electron donor-property compound and an electron acceptor-property compound in its electron injection layer but does not provide good light emission. The reason for the foregoing is described below.

In general, there is a large energy barrier between a cathode and an organic compound layer (such as an electron transport layer, a hole blocking layer, or an emission layer). The HOMO of the electron donor-property compound (Compound a-1) disclosed in U.S. Patent Application Publication No. 2005/0110005 and the LUMO of the electron acceptor-property compound (Compound a-2) disclosed therein are each −4.5 eV, which is substantially equal to the work function (4.3 eV) of aluminum that has been generally used as a cathode. Therefore, an energy barrier with respect to, for example, the LUMO (−3.3 eV) of Alq₃ that has been normally used as a light emitting material in an organic light emitting element enlarges. Accordingly, even when the DA complex described in U.S. Patent Application Publication No. 2005/0110005 is introduced into a layer (an electron transport layer or an electron injection layer) having an electron injection/transport property constituting an organic light emitting element, a voltage needed for driving the element increases, and as a result, good light emission from its emission layer is not obtained.

In contrast, the electron donor-property compound (viologen compound) to be used as a constituent material for the electron injection layer in the organic light emitting element of the present invention has the following advantages: the compound has a high electron donor property, has a low first oxidation potential, and has a shallow HOMO (in a direction approaching a vacuum level).

Here, the following compounds are considered from the viewpoint of an oxidation-reduction potential on the assumption that there is a correlation between a first oxidation potential and a HOMO, and there is also a correlation between a first reduction potential and a LUMO.

The compounds were subjected to CV measurement under the same conditions. As a result, the first oxidation potentials of Compound a-1 and Compound B1 were 0.3 V and −1.2 V, respectively. The foregoing shows that Compound B1 has a higher electron donor property because the first oxidation potential of Compound B1 is smaller than that of Compound a-1 by 1.5 V. Therefore, when the viologen compound represented by the general formula [1] is adopted as an electron donor-property compound to establish a state where a DA complex is formed in the electron injection layer, the ability to inject an electron into the organic compound layer (such as the electron transport layer, the hole blocking layer, or the emission layer) increases. As a result, good light emission from the emission layer is obtained.

In the organic light emitting element of the present invention, the electron injection layer includes: a mixed film formed of the electron donor-property compound X (viologen compound) and the electron acceptor-property compound Y; and a stacked film obtained by stacking the electron donor-property compound X (viologen compound) and the electron acceptor-property compound Y. Accordingly, in any case, the transfer of charge from the electron donor-property compound X to the electron acceptor-property compound Y or the production of a DA complex polarized by a strong interaction between both the compounds occurs. Here, such interaction occurs between the HOMO of the electron donor-property compound X and the LUMO of the electron acceptor-property compound Y. In addition, in the case where the electron injection layer is of a stacked structure, charge occurs between the electron acceptor-property compound and the electron donor-property compound, and hence an electron is easily accepted from the electrode (cathode). On the other hand, in the case of a mixed layer, the DA complex has a carrier and hence the charge mobility of the layer itself increases. Therefore, the electron injection layer constituting the organic light emitting element of the present invention is excellent in electron transport ability as well as in electron injection ability.

In addition, with regard to the relationship of the DA complex, it has been generally known that as the oxidation potential of the electron donor-property compound X and the reduction potential of the electron acceptor-property compound Y become closer to each other, a stronger interaction occurs between the D and A molecules.

Therefore, specifically, as represented by the general formula [4], the potential difference (|V_(red)−V_(ox)|) obtained by CV measurement is preferably within 1.0 V. The potential difference is more preferably within 0.5 V as represented by the general formula [5]. This is because additionally reducing the potential difference additionally strengthens the interaction between the D and A molecules.

Compound b-1 shown below, which is disclosed in J. Org. Chem. 2008, 73, 445-450, is a viologen compound having a high electron donor property.

However, Compound b-1 has a large molecular weight and hence it is difficult to subject the compound to sublimation purification. No examples of the application of Compound b-1 to an electronic device such as an organic light emitting element have been shown probably because of the foregoing. In general, a compound to be used as a constituent material for an organic light emitting element needs to have a high purity in order that the durability of the element may be improved. Therefore, whether a compound of interest can be subjected to sublimation purification as (one of) the methods of increasing the purity of the compound itself is important.

In this respect, the electron donor-property compound X represented by the general formula [1] (viologen compound) has a molecular weight smaller than that of Compound b-1 and hence can be subjected to sublimation purification.

By the way, an organic compound having a high electron donor property and a small first oxidation potential generally tends to be readily oxidized in the air. However, the viologen compound to be used as the electron donor-property compound X is a compound that stably exists without being quickly oxidized even in the air. Therefore, the organic light emitting element of the present invention is stable even against a factor involved in the degradation of the organic light emitting element such as oxygen in the air despite the fact that the element contains the compound having a high electron donor property.

As described above, the electron injection layer constituting the organic light emitting element of the present invention can satisfactorily perform the injection of an electron from the cathode, the transport of an electron, and the injection of an electron into an organic compound layer (such as the electron transport layer, the hole blocking layer, or the emission layer). In addition, an organic light emitting element that is stable in the air can be provided by: introducing the electron donor-property compound X represented by the general formula [1] (viologen compound) and the electron acceptor-property compound Y into the electron injection layer; and satisfying the formula [4] between the compound X and the compound Y.

[Specific Examples of Electron Donor-Property Compound X]

Specific examples of the electron donor-property compound X (viologen compound) to be used as a constituent material for the organic light emitting element of the present invention are shown below. However, the present invention is not limited to these specific examples.

[Properties of Exemplified Compounds (Electron Donor-Property Compound X)]

Hereinafter, the compounds listed as the specific examples of the electron donor-property compound X are described.

By the way, any one of the physical properties of the electron donor-property compound X itself, specifically its oxidation potential, film property, heat stability, and sublimation property can be finely adjusted depending on the kinds and number of substituents to be introduced into a viologen skeleton as the basic skeleton of the compound. Those physical properties can be appropriately selected depending on use purposes.

Each of the exemplified compounds is a compound having a high electron donor property because all the compounds have viologen basic skeletons. Accordingly, a compound that improves stability against the air is preferred. In the present invention, in particular, the following two kinds of compounds are given as a group of compounds that improve stability against the air. It is to be noted that the compound represented by the following general formula [6] includes compounds belonging to the C group or the D group out of the listed exemplified compounds, and the compound represented by the following general formula [7] includes the compounds belonging to the B group out of the listed exemplified compounds.

In the general formula [6], Ar₁ and Ar₂ each represent an aryl group.

Examples of the aryl group represented by Ar₁ or Ar₂ include a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, an anthracenyl group, a fluorenyl group, a phenanthryl group, a benzophenanthryl group, a chrysenyl group, and a fluoranthenyl group.

In addition, the aryl group may further have a fluorine atom, an alkyl group such as a methyl group, an ethyl group, an isopropyl group, or a tert-butyl group, an alkoxy group such as a methoxy group, or an amino group.

The compound represented by the general formula [6] is preferred because of the following reason: the planarity of its viologen main skeleton is improved and the intermolecular stacking property of the compound is improved by substituting the main skeleton with an aryl group having a high stacking effect as a substituent, and hence the penetration of the air can be suppressed.

In the general formula [7], R₂₀₁ to R₂₀₆ each represent a substituent selected from the group consisting of a fluorine atom, an alkyl group, an alkoxy group, and an aryl group.

Examples of the alkyl group represented by any one of R₂₀₁ to R₂₀₆ include a methyl group, an ethyl group, a n-propyl group, an isopropyl group, a n-butyl group, and a tert-butyl group.

An example of the alkoxy group represented by any one of R₂₀₁ to R₂₀₆ is a methoxy group.

Examples of the aryl group represented by any one of R₂₀₁ to R₂₀₆ include a phenyl group, a biphenyl group, a terphenyl group, a naphthyl group, an anthracenyl group, a fluorenyl group, a phenanthryl group, a benzophenanthryl group, a chrysenyl group, and a fluoranthenyl group.

In addition, the aryl group may further have a fluorine atom, an alkyl group such as a methyl group, an ethyl group, an isopropyl group, or a tert-butyl group, an alkoxy group such as a methoxy group, or an amino group.

In the compound represented by the general formula [7], a carbon atom bonded to a nitrogen atom of its viologen skeleton is substituted with any one of a fluorine group, an alkyl group, an alkoxy group, and an aryl group as substituents each having high hydrophobicity. Such substitution is preferred because a substitution position that is liable to be oxidized is capped and hence the oxidation of the compound can be suppressed. This is because nitrogen of a tertiary amine has a high electron-donating property and the reactivity of an sp² carbon atom that has additionally received the effect improves.

In addition, out of the compounds each represented by the general formula [7], a compound represented by the following general formula [8] is preferred.

In the general formula [8], all of R₂₀₃ to R₂₀₆ represent aryl groups, and R₂₁₁ to R₂₂₀ each represent a hydrogen atom or a substituent selected from the group consisting of a fluorine atom, an alkyl group, an alkoxy group, and an aryl group.

It is to be noted that the aryl group may further have a fluorine atom, an alkyl group such as a methyl group, an ethyl group, an isopropyl group, or a tert-butyl group, an alkoxy group such as a methoxy group, or an amino group.

When all of R₂₀₃ to R₂₀₆ represent aryl groups as represented by the general formula [8], R₂₀₁ and R₂₀₂ in the general formula [7] each preferably represent a phenyl group. This is because of the following reason: when all of R₂₀₃ to R₂₀₆ represent aryl groups, the molecular weight of the compound increases and the compound becomes sterically bulky, and hence R₂₀₁ and R₂₀₂ in the general formula [7] each preferably represent a phenyl group, which is smallest among the aryl groups, from the viewpoints of sublimation purification and steric bulkiness.

[Specific Examples of Electron Acceptor-Property Compound]

Specific examples of the electron acceptor-property compound to be used as a constituent material for the organic light emitting element of the present invention are shown below. However, the present invention is not limited to these specific examples.

[Other Constituent Material for Organic Light Emitting Element]

In the organic light emitting element of the present invention, in addition to the electron donor-property compound X and the electron acceptor-property compound Y, a known compound can be used as a constituent material to be incorporated into the element. Examples of such compound are given below.

A material having a high hole mobility is preferred as a hole injectable/transportable material so that the injection of a hole from the anode may be facilitated and the injected hole can be transported to the emission layer. In addition, a material having a high glass transition temperature is preferred in order that the degradation of film quality such as crystallization in the organic light emitting element may be prevented. A triarylamine derivative, an arylcarbazole derivative, a phenylenediamine derivative, a stilbene derivative, a phthalocyanine derivative, a porphyrin derivative, poly(vinyl carbazole), poly(thiophene), and any other conductive polymer are given as a low-molecular material or high-molecular material having hole injection/transport performance. Further, the hole injectable/transportable material is also suitably used for an electron blocking layer.

Specific examples of the compound to be used as the hole injectable/transportable material are shown below. However, the compound is of course not limited to the following.

As a light emitting material mainly involved in light emitting function among constituent materials for the emission layer, there are given, for example, condensed ring compounds (such as a fluorene derivative, a naphthalene derivative, a pyrene derivative, a perylene derivative, a tetracene derivative, an anthracene derivative, and rubrene), a quinacridone derivative, a coumarin derivative, a stilbene derivative, an organic aluminum complex such as tris(8-quinolinolato)aluminum, an iridium complex, a platinum complex, a rhenium complex, a copper complex, a europium complex, a ruthenium complex, and polymer derivatives such as a poly(phenylene vinylene) derivative, a poly(fluorene) derivative, and a poly(phenylene) derivative.

Specific examples of the compound to be used as the light emitting material are shown below. However, the compound is of course not limited to the following.

As a host or an assist material (emission assist material) to be incorporated into the emission layer, there are given, for example, an aromatic hydrocarbon compound and a derivative thereof as well as a carbazole derivative, a dibenzofuran derivative, a dibenzothiophene derivative, an organic aluminum complex such as a tris(8-quinolinolato)aluminum, and an organic beryllium complex.

Specific examples of the compound to be used as the host (emission layer host) or emission assist material to be incorporated into the emission layer are shown below. However, the compound is of course not limited to the following.

An electron transportable material can be arbitrarily selected from materials that can transport an electron injected from the cathode to the emission layer, and the material is appropriately selected in consideration of, for example, the balance with the hole mobility of the hole transportable material. Examples of the material having electron transport performance include an oxadiazole derivative, an oxazole derivative, a pyrazine derivative, a triazole derivative, a triazine derivative, a quinoline derivative, a quinoxaline derivative, a phenanthroline derivative, an organic aluminum complex, and condensed ring compounds (such as a fluorene derivative, a naphthalene derivative, a chrysene derivative, and an anthracene derivative). Further, the electron transportable material is also suitably used for the hole blocking layer.

Specific examples of the compound to be used as the electron transportable material are shown below. However, the compound is of course not limited to the following.

An electron injectable material can be arbitrarily selected from materials that can facilitate the injection of an electron from the cathode, and the material is selected in consideration of, for example, balance with a hole injection property. The electron injectable material includes the electron donor-property compound (viologen compound) that has already been described, an n-type dopant, and a reducing dopant as well. In addition to the viologen compound, examples of such material having electron injection performance include an alkali metal, an alkaline earth metal, a rare earth metal, an oxide of an alkali metal, a halide of an alkali metal, a carbonate of an alkali metal, an alkali metal complex, an oxide of an alkaline earth metal, a halide of an alkaline earth metal, an alkaline earth metal complex, an oxide of a rare earth metal, a halide of a rare earth metal, and a rare earth metal complex.

A constituent material for the anode desirably has as large a work function as possible. Examples thereof may include: metal simple substances such as gold, platinum, silver, copper, nickel, palladium, cobalt, selenium, vanadium, and tungsten or alloys obtained by combining these metal simple substances; metal oxides such as tin oxide, zinc oxide, indium oxide, indium tin oxide (ITO), and indium zinc oxide; and conductive polymers such as polyaniline, polypyrrole, and polythiophene.

One kind of those electrode substances may be used alone, or two or more kinds thereof may be used in combination. In addition, the anode may be of a single-layer construction or may be of a multilayer construction.

On the other hand, a constituent material for the cathode is desirably one having as small a work function as possible, but is not limited thereto. Examples thereof include: alkali metals such as lithium; alkaline earth metals such as calcium; and metal simple substances such as aluminum, titanium, manganese, silver, lead, chromium, and gold. Alternatively, alloys obtained by combining those metal simple substances can be used. For example, a magnesium-silver alloy, an aluminum-lithium alloy, or an aluminum-magnesium alloy can be used. A metal oxide such as indium tin oxide (ITO) can also be utilized. One kind of those electrode substances may be used alone, or two or more kinds thereof may be used in combination. In addition, the cathode may be of a single-layer construction or may be of a multilayer construction.

In the organic light emitting element of the present invention, a light extraction construction may be a bottom emission construction (construction in which light is extracted from a substrate side) or may be a top emission construction (construction in which light is extracted from a side opposite to the substrate). In the case of the bottom emission construction, while a first electrode to be formed on the substrate side is a light transmissive electrode or light semi-transmissive electrode, a second electrode to be formed on the side opposite to the substrate is a light reflective electrode. In addition, in the case of the top emission construction, while the first electrode is the light reflective electrode, the second electrode is the light transmissive electrode or light semi-transmissive electrode. A metal conductive layer having a large thickness (of 80 nm or more and 600 nm or less) can be used as the light reflective electrode. In addition, a metal conductive layer having a small thickness (of from 15 nm to 35 nm) can be used as the light semi-transmissive electrode.

The organic light emitting element of the present invention is preferably sealed in order that the contact of oxygen, moisture, or the like may be suppressed. A method of sealing the organic light emitting element of the present invention is, for example, a method involving forming a sealing layer on the organic light emitting element. Examples of a constituent material for the sealing layer for sealing the organic light emitting element include materials such as: inorganic oxides, inorganic nitrides, and inorganic oxynitrides such as SiO₂, SiN, and SiON; polymer compounds such as a fluorine resin, polyparaxylene, polyethylene, a silicone resin, and a polystyrene resin; and photocurable resins. In addition, a member formed by an atomic layer deposition (ALD) method on the second electrode can be used as the sealing layer.

In addition, an organic film layer such as an optical interference layer can be formed on the second electrode.

The organic compound layer (such as the hole injection layer, the hole transport layer, the electron blocking layer, the emission layer, the hole blocking layer, the electron transport layer, or the electron injection layer) for forming the organic light emitting element of the present invention is formed by the following method.

A dry process such as a vacuum vapor deposition method, an ionized vapor deposition method, sputtering, or a plasma process can be used for the formation of the organic compound layer for forming the organic light emitting element of the present invention. In addition, a wet process involving dissolving the constituent materials in an appropriate solvent and forming a layer by a known application method (such as spin coating, dipping, a casting method, an LB method, or an ink jet method) can be used instead of the dry process.

Here, when the layer is formed by the vacuum vapor deposition method, the solution application method, or the like, the layer hardly undergoes crystallization or the like and is excellent in stability over time. In addition, when the layer is formed by the application method, the film can be formed by using the constituent materials in combination with an appropriate binder resin.

Examples of the binder resin include, but not limited to, a polyvinyl carbazole resin, a polycarbonate resin, a polyester resin, an ABS resin, an acrylic resin, a polyimide resin, a phenol resin, an epoxy resin, a silicone resin, and a urea resin.

In addition, one kind of those binder resins may be used alone as a homopolymer or a copolymer, or two or more kinds thereof may be used as a mixture. Further, a known additive such as a plasticizer, an antioxidant, or a UV absorber may be used in combination as required.

[Application of Organic Light Emitting Element]

The organic light emitting element of the present invention can be used as a constituent member for a display apparatus or lighting apparatus. In addition, the element finds use in applications such as an exposure light source for an image forming apparatus of an electrophotographic system, a backlight for a liquid crystal display apparatus, and a light emitting apparatus including a white light source and a color filter. Examples of the color filter include filters that transmit light beams having three colors, i.e., red, green, and blue colors.

A display apparatus of the present invention includes the organic light emitting element of the present invention in its display portion. It is to be noted that the display portion includes a plurality of pixels.

In addition, the pixels each include the organic light emitting element of the present invention and a transistor as an example of an active element (switching element) or amplifying element configured to control emission luminance, and the anode or cathode of the organic light emitting element and the drain electrode or source electrode of the transistor are electrically connected to each other. Here, the display apparatus can be used as an image display apparatus for a PC or the like. The transistor is, for example, a TFT element and the TFT element is provided on, for example, the insulating surface of a substrate.

The display apparatus may be an image information processing apparatus that includes an image input portion configured to input image information from, for example, an area CCD, a linear CCD, or a memory card, and displays an input image on its display portion.

In addition, the display portion of an imaging apparatus or inkjet printer may have a touch panel function. The drive system of the touch panel function is not particularly limited.

In addition, the display apparatus may be used in the display portion of a multifunction printer.

A lighting apparatus is an apparatus configured to light, for example, the inside of a room. The lighting apparatus may emit light having any one of the following colors: a white color (having a color temperature of 4,200 K), a daylight color (having a color temperature of 5,000 K), and colors ranging from blue to red colors.

A lighting apparatus of the present invention includes the organic light emitting element of the present invention and an AC/DC converter circuit (circuit configured to convert an AC voltage into a DC voltage) connected to the organic light emitting element and configured to supply a driving voltage to the organic light emitting element. It is to be noted that the lighting apparatus may further include a color filter.

An image forming apparatus of the present invention is an image forming apparatus including: a photosensitive member; a charging unit configured to charge the surface of the photosensitive member; an exposing unit configured to expose the photosensitive member to form an electrostatic latent image; and a developing device configured to develop the electrostatic latent image formed on the surface of the photosensitive member. Here, the exposing unit to be provided in the image forming apparatus includes the organic light emitting element of the present invention.

In addition, the organic light emitting element of the present invention can be used as a constituent member for an exposing apparatus configured to expose a photosensitive member. An exposing apparatus including a plurality of the organic light emitting elements of the present invention is, for example, an exposing apparatus in which the organic light emitting elements of the present invention are placed to form a line along a predetermined direction.

Next, the display apparatus of the present invention is described with reference to the drawings. FIG. 1 is a schematic sectional view illustrating an example of a display apparatus including an organic light emitting element and a TFT element connected to the organic light emitting element. It is to be noted that the organic light emitting element of the present invention is used as the organic light emitting element constituting a display apparatus 1 of FIG. 1.

The display apparatus 1 of FIG. 1 includes a substrate 11 made of glass or the like and a moisture-proof film 12 for protecting a TFT element or organic compound layer, the film being formed on the substrate. In addition, a metal gate electrode 13 is represented by reference numeral 13, a gate insulating film 14 is represented by reference numeral 14, and a semiconductor layer is represented by reference numeral 15.

A TFT element 18 includes the semiconductor layer 15, a drain electrode 16, and a source electrode 17. An insulating film 19 is formed on the TFT element 18. An anode 21 constituting the organic light emitting element and the source electrode 17 are connected to each other through a contact hole 20.

It is to be noted that a system for the electrical connection between the electrode (anode or cathode) in the organic light emitting element and the electrode (source electrode or drain electrode) in the TFT is not limited to the aspect illustrated in FIG. 1. In other words, one of the anode and the cathode, and one of the source electrode and drain electrode of the TFT element only need to be electrically connected to each other.

Although a plurality of organic compound layers are illustrated like one layer in the display apparatus 1 of FIG. 1, an organic compound layer 22 may be a plurality of layers. A first protective layer 24 and second protective layer 25 for suppressing the degradation of the organic light emitting element are formed on a cathode 23.

When the display apparatus 1 of FIG. 1 is a display apparatus that emits white light, an emission layer in the organic compound layer 22 in FIG. 1 may be a layer obtained by mixing a red light emitting material, a green light emitting material, and a blue light emitting material. In addition, the layer may be a stacked emission layer obtained by stacking a layer formed of the red light emitting material, a layer formed of the green light emitting material, and a layer formed of the blue light emitting material. Further, alternatively, the following aspect is permitted: the layer formed of the red light emitting material, the layer formed of the green light emitting material, and the layer formed of the blue light emitting material are, for example, arranged side by side to form domains in one emission layer.

Although the transistor is used as the switching element in the display apparatus 1 of FIG. 1, an MIM element may be used instead of the transistor as the switching element.

In addition, the transistor to be used in the display apparatus 1 of FIG. 1 is not limited to a transistor using a monocrystalline silicon wafer and may be a thin-film transistor including an active layer on the insulating surface of a substrate. In addition, a thin-film transistor using monocrystalline silicon as the active layer, a thin-film transistor using non-monocrystalline silicon such as amorphous silicon or microcrystalline silicon as the active layer, a thin-film transistor using a non-monocrystalline oxide semiconductor such as indium zinc oxide or indium gallium zinc oxide as the active layer, an organic transistor using a film formed of an organic material as the active layer, or the like can also be used. It is to be noted that the thin-film transistor is also called a TFT element.

The channel portion of the switching element (active element) according to this embodiment may contain an oxide semiconductor. In the switching element, the oxide semiconductor in the channel portion may be in an amorphous state or may be in a crystalline state. Alternatively, the semiconductor may be in such a state that both the states (the amorphous and crystalline states) are mixed. Here, when the oxide semiconductor in the channel portion is in a crystalline state, a specific aspect of the crystal is, for example, a single crystal, a microcrystal, or a crystal whose specific axis such as a C-axis is oriented. However, the specific aspect of the crystal is not limited to one kind of the aspects, and an aspect obtained by combining at least a plurality (e.g., two kinds) of the aspects is also naturally included. The organic light emitting element including such switching element may be used in an image display apparatus in which each organic light emitting element is provided as a pixel, or may be used as a lighting apparatus or as an exposure portion for exposing a photosensitive member of an image forming apparatus of an electrophotographic system such as a laser beam printer or a copying machine.

The transistor in the display apparatus 1 of FIG. 1 may be formed in a substrate such as a Si substrate. Here, the phrase “formed in a substrate” means that the transistor is produced by processing the substrate itself such as a Si substrate. In other words, the presence of the transistor in the substrate can be regarded as follows: the substrate and the transistor are integrally formed.

Whether the transistor is provided in the substrate is selected depending on definition. In the case of, for example, a definition of about a QVGA per inch, the organic light emitting element is preferably provided in the Si substrate.

As described above, the driving of the display apparatus using the organic light emitting element of the present invention enables display that has good image quality and is stable over a long time period.

Next, other applications of the organic light emitting element of the present invention are described. FIG. 2 is a schematic view illustrating an example of an image forming apparatus including the organic light emitting element according to the present invention. An image forming apparatus 26 of FIG. 2 includes a photosensitive member 27, an exposure light source 28, a developing device 30, a charging portion 31, a transferring device 32, a conveying roller 33, and a fixing device 35.

In the image forming apparatus 26 of FIG. 2, light 29 is applied from the exposure light source 28 to the photosensitive member 27, whereby an electrostatic latent image is formed on the surface of the photosensitive member 27. In the image forming apparatus 26 of FIG. 2, the exposure light source 28 includes the organic light emitting element according to the present invention. In addition, in the image forming apparatus 26 of FIG. 2, the developing device 30 as a developing portion has a developer such as toner and is configured to apply the developer to the photosensitive member 27. In the image forming apparatus 26 of FIG. 2, the charging portion 31 is provided for charging the photosensitive member 27. In the image forming apparatus 26 of FIG. 2, the transferring device 32 is provided for transferring a developed image onto a recording medium 34 such as paper. It is to be noted that the recording medium 34 is conveyed by the conveying roller 33 to the transferring device 32. In the image forming apparatus 26 of FIG. 2, the fixing device 35 is provided for fixing the image formed on the recording medium 34.

FIG. 3A and FIG. 3B are each a schematic plan view illustrating a specific example of the exposure light source constituting the image forming apparatus 26 of FIG. 2, and FIG. 3C is a schematic view illustrating a specific example of the photosensitive member constituting the image forming apparatus 26 of FIG. 2. It is to be noted that FIG. 3A and FIG. 3B have the following feature in common: a plurality of emission portions 36 (emission points) each including the organic light emitting element are placed on an elongated substrate 28 a of the exposure light source 28. In addition, an arrow represented by reference numeral 37 represents a column direction in which the emission portions 36 are arranged. The column direction is the same as the direction of the axis about which the photosensitive member 27 rotates.

By the way, FIG. 3A illustrates a form in which the plurality of emission portions 36 of the exposure light source 28 are placed along the long axis direction of the photosensitive member 27. On the other hand, FIG. 3B illustrates a form in which the emission portions 36 are alternately placed in the column direction in a first column α and a second column β. In FIG. 3B, the first column α and the second column β are placed at different positions in a row direction.

In addition, in FIG. 3B, while a plurality of emission portions 36α are placed at a certain interval in the first column α, the second column β has an emission portion 36β at a position corresponding to an interval between the emission portions 36α in the first column α. That is, in the exposure light source of FIG. 3B, the plurality of emission portions are placed at a certain interval in the row direction as well.

It is to be noted that the following rewording is permitted: the exposure light source of FIG. 3B is in a state where the emission portions (36α and 36β) constituting the exposure light source are placed in, for example, a lattice, hound's-tooth, or checkered pattern.

FIG. 4 is a schematic view illustrating an example of a lighting apparatus including the organic light emitting element according to the present invention. The lighting apparatus of FIG. 4 includes an organic light emitting element 38 provided on a substrate (not shown) and an AC/DC converter circuit 39. In addition, the lighting apparatus of FIG. 4 may include a heat sink (not shown) corresponding to a heat discharging portion for discharging heat in the apparatus to the outside on, for example, a substrate surface on a side opposite to the side on which the organic light emitting element 38 is mounted.

EXAMPLES

Hereinafter, the present invention is described by way of Examples. However, the present invention is not limited to Examples described below.

[Electron Donor-Property Compound X (Viologen Compound)]

The viologen compound as the electron donor-property compound X used in the production of the organic light emitting element of the present invention was synthesized by an existing method. Specifically, as represented by the following reaction formula, the viologen compound was obtained by reducing a halide salt as a precursor of the compound.

Available as a method of reducing the halide salt is, for example, (i) a reduction method involving using zinc or (ii) a reduction method involving using a hyposulfite ion. Here, the methods (i) and (ii) are described by taking the synthesis of Exemplified Compound C1 as a specific example. It is to be noted that the synthesis scheme of Exemplified Compound C1 is as shown below.

<Reduction Method Involving Using Zinc (Method (i))>

Compound C1-pre, zinc powder, and ethanol were loaded into a reaction vessel, and then the reaction solution was heated to reflux for 8 hours. Thus, Exemplified Compound C1 was obtained.

<Reduction Method Involving Using Hyposulfite Ion (Method (ii))>

Compound C1-pre, 10% ammonia water, sodium hyposulfite, and ethanol were loaded into a reaction vessel, and then the reaction solution was stirred at room temperature for 1 hour. Thus, Exemplified Compound C1 was obtained.

[CV Measurement]

A halide salt as a precursor of the electron donor-property compound X and the electron acceptor-property compound Y were subjected to CV measurement. Specific measurement conditions are listed below.

Measurement environment (gas atmosphere): nitrogen atmosphere

Electrolytic solution: 0.1 M tetrabutylammonium perchlorate solution in DMF

Reference electrode: Ag/Ag⁺

Counter electrode: Pt

Working electrode: glassy carbon

Measurement apparatus: electrochemical analyzer (manufactured by ALS, model 660C)

Sweeping rate: 1.0 V/s

Table 2 shows the first reduction potential value (V_(red)) of the electron acceptor-property compound Y and the first oxidation potential value (V_(ox)) of the electron donor-property compound X obtained under the measurement conditions. It is to be noted that in each of the V_(red) and the V_(ox), in consideration of a measurement error, when the second decimal place of a measured value was 0, 1, 2, 8, or 9, the second decimal place was set to 0, and when the second decimal place of the measured value was 3, 4, 5, 6, or 7, the second decimal place was set to 5.

Example 1

In this example (Example 1), an organic light emitting element of a bottom emission construction in which an anode, a hole transport layer, an electron blocking layer, an emission layer, an electron transport layer, an electron injection layer, and a cathode were formed in the stated order on a substrate was produced by a method to be described below.

Indium tin oxide (ITO) was formed into a film on a glass substrate by a sputtering method. Thus, the anode was formed. At this time, the thickness of the anode was set to 120 nm. Next, the substrate having formed thereon the anode was sequentially subjected to ultrasonic washing with acetone and isopropyl alcohol (IPA), and was then subjected to boil washing with IPA, followed by drying. Further, the dried product was subjected to UV/ozone washing. The substrate treated by the foregoing method was used as a transparent conductive supporting substrate (ITO substrate) in the next step.

Next, layers and an electrode layer shown in Table 1 below were continuously formed on the ITO substrate by vacuum deposition based on resistance heating in a vacuum chamber at 10⁻⁵ Pa. At this time, the layers were formed so that an opposing electrode area became 3 mm2.

TABLE 1 Thickness Constituent material [nm] Hole transport HT2 50 layer Electron HT7 10 blocking layer Emission layer Host: EM12 30 Guest: RD1 (Host:guest = 99.5:0.5 (weight ratio)) Electron ET6 50 transport layer Electron C1 (Electron donor-property 15 injection layer compound), E10 (Electron acceptor-property compound) (C1:E10 = 40:60 (weight ratio)) Metal electrode Al 100 layer (cathode)

The resultant organic light emitting element was driven so that its luminance became 100 cd/m². As a result, its emission efficiency was 1.3 cd/A and red light emission was observed.

Examples 2 to 7

Organic light emitting elements were each produced by the same method as that of Example 1 with the exception that in Example 1, the constituent materials for the electron injection layer were changed as shown in Table 2 below. The resultant organic light emitting elements were evaluated for their element characteristics by the same method as that of Example 1. Table 2 shows the results.

Comparative Example 1

An organic light emitting element was produced by the same method as that of Example 1 with the exception that in Example 1, Compound a-1 shown below was used as an electron donor-property compound instead of Exemplified Compound C1. The resultant organic light emitting element was evaluated for its element characteristic by the same method as that of Example 1. Table 2 shows the result.

TABLE 2 Electron injection layer V_(ox) − Donor Acceptor V_(red) Efficiency V_(ox) (V) V_(red) (V) (V) (cd/A) Example 1 C1 −1.10 E10 −0.90 −0.20 1.3 Example 2 C1 −1.10 E7 −1.20 0.10 0.9 Example 3 C3 −1.20 E10 −0.90 −0.30 1.4 Example 4 C3 −1.20 E7 −1.20 0 1.0 Example 5 C8 −0.90 E5 −0.90 0 0.9 Example 6 B5 −1.20 E10 −0.90 −0.30 1.2 Example 7 B5 −1.20 E7 −1.20 0 0.8 Comparative a-1 0.30 a-2 −0.90 1.20 No light Example 1 (E10) emission

Examples 8 to 11

Organic light emitting elements were each produced by the same method as that of Example 1 with the exception that in Example 1, the constituent materials for the emission layer were changed as shown in Table 3 below. The resultant organic light emitting elements were evaluated for their element characteristics by the same method as that of Example 1. Table 3 shows the results.

TABLE 3 Emission layer Weight Dop- Weight Efficiency Luminescent Host ratio ant ratio (cd/A) color Example 8 EM3 95% BD6 5% 0.3 Blue light emission Example 9 EM2 98% GD4 2% 1.8 Green light emission Example EM10 96% RD3 4% 2.8 Red light 10 emission Example EM7 90% GD6 10% 3.7 Green light 11 emission

Examples 12 to 15

Organic light emitting elements were each produced by the same method as that of Example 1 with the exception that in Example 1, the constituent materials for the electron injection layer were changed as shown in Table 4 below. The resultant organic light emitting elements were evaluated for their element characteristics by the same method as that of Example 1. Table 4 shows the results.

TABLE 4 Electron injection layer V_(ox) − Donor Acceptor V_(red) Efficiency V_(ox) (V) V_(red) (V) (V) (cd/A) Example 12 C9 −1.00 E8 −1.95 0.95 1.1 Example 13 C9 −1.00 E20 −1.80 0.80 1.6 Example 14 D6 −0.90 E19 −1.85 0.95 1.5 Example 15 D6 −0.90 E20 −1.80 0.90 1.4

Example 16

An organic light emitting element of a bottom emission construction as described below in which an anode, a hole transport layer, an electron blocking layer, an emission layer, a hole blocking layer, an electron injection/transport layer, and a cathode were formed in the stated order on a substrate was produced. It is to be noted that the electron injection/transport layer constituting the organic light emitting element of this example is a layer that brings together the functions of an electron injection layer and an electron transport layer.

Layers and an electrode layer shown in Table 5 below were continuously formed on the ITO substrate produced in Example 1 by vacuum deposition based on resistance heating in a vacuum chamber at 10⁻⁵ Pa. At this time, the layers were formed so that an opposing electrode area became 3 mm².

TABLE 5 Thickness Constituent material [nm] Hole transport layer HT2 50 Electron blocking layer HT7 10 Emission layer Host: EM14 20 Guest: RD1 (Host:guest = 99.5:0.5 (volume ratio)) Hole blocking layer EM15 10 Electron E19, C1 53 injection/transport layer ^((Note 1)) Metal electrode layer Aluminum 120 (Cathode)

The resultant organic light emitting element was driven so that its luminance became 100 cd/m². As a result, its emission efficiency was 3.6 cd/A and red light emission was observed.

Examples 17 to 19 and Comparative Example 2

Organic light emitting elements were each produced by the same method as that of Example 16 with the exception that in Example 16, the constituent materials for the electron injection/transport layer were changed as shown in Table 6 below. The resultant organic light emitting elements were evaluated for their element characteristics by the same method as that of Example 16. Table 6 shows the results.

TABLE 6 Electron injection/transport layer Acceptor layer Donor layer V_(ox) − V_(red) Efficiency V_(red) (V) V_(ox) (V) (V) (cd/A) Example 16 E19 −1.85 C1 −1.10 0.75 3.6 Example 17 E20 −1.80 C1 −1.10 0.70 3.5 Example 18 E19 −1.85 C9 −0.90 0.95 1.9 Example 19 E20 −1.80 C9 −0.90 0.90 2.0 Comparative E5 −2.30 No light Example 2 emission

Example 20

In this example, an organic light emitting element was produced by the same method as that of Example 16 with the exception that in Example 16, the constituent material for the metal electrode layer (cathode) was changed to silver.

The resultant organic light emitting element was driven so that its luminance became 100 cd/m². As a result, its emission efficiency was 4.2 cd/A and red light emission was observed.

Examples 21 to 23

Organic light emitting elements were each produced by the same method as that of Example 20 with the exception that in Example 20, the constituent materials for the electron injection/transport layer were changed as shown in Table 7 below. The resultant organic light emitting elements were evaluated for their element characteristics by the same method as that of Example 20. Table 7 shows the results.

TABLE 7 Electron injection (and electron transport) layer Acceptor layer Donor layer V_(ox) − V_(red) Efficiency V_(red) (V) V_(ox) (V) (V) (cd/A) Example 20 E19 −1.85 C1 −1.10 0.75 4.2 Example 21 E20 −1.80 C1 −1.10 0.70 4.1 Example 22 E19 −1.85 C9 −0.90 0.95 2.5 Example 23 E20 −1.80 C9 −0.90 0.90 2.7 Comparative E5 −2.30 No light Example 3 emission

Example 24

In this example, an organic light emitting element of a top emission construction in which a first electrode (anode), a hole transport layer, an electron blocking layer, an emission layer, a hole blocking layer, an electron injection/transport layer, and a second electrode (cathode) were formed in the stated order on a substrate was produced by a method to be described below.

An aluminum metal and indium tin oxide (ITO) were formed into films and stacked in the stated order on a glass substrate (substrate) by a sputtering method. Thus, the first electrode as an anode was formed. At this time, the thickness of the aluminum film constituting the first electrode was set to 100 nm and the thickness of the ITO film constituting the first electrode was set to 30 nm. Next, the substrate having formed thereon the first electrode was sequentially subjected to ultrasonic washing with acetone and isopropyl alcohol (IPA), and was then subjected to boil washing with IPA, followed by drying. Further, the dried product was subjected to UV/ozone washing, to thereby obtain an ITO substrate.

Next, layers and the second electrode as a cathode shown in Table 8 below were continuously formed on the ITO substrate by vacuum deposition based on resistance heating in a vacuum chamber at 10⁻⁵ Pa. At this time, the layers were formed so that an emission area in which the first electrode and the second electrode overlapped each other became 4 mm².

TABLE 8 Thickness Constituent material [nm] Hole transport layer HT2 200 Electron blocking HT7 10 layer Emission layer Host: EM14 20 Guest: RD1 (Host:guest = 99.5:0.5 (volume ratio)) Hole blocking layer EM15 10 Electron E19, C1 33 injection/transport layer ^((Note 1)) Second electrode Aluminum, silver 30 (cathode) ^((Note 2)) ^((Note 1)) The electron injection/transport layer is a stack in which E19 (30 nm, acceptor layer) and C1 (3 nm, donor layer) are stacked in the stated order. In addition, V_(ox)− V_(red) in Example 24 is 0.75 V. ^((Note 2)) The second electrode is a stack in which aluminum (5 nm) and silver (25 nm) are stacked in the stated order.

It is to be noted that in the organic light emitting element produced in this example, the second electrode was a light semi-transmissive electrode, and was a stacked electrode formed of aluminum (5 nm) and silver (25 nm) in consideration of a reflectance, absorption, and a conductivity. In addition, a cavity construction in which light extraction efficiency was improved by utilizing optical interference was adopted, and the thicknesses of the respective layers were determined so that their interference conditions coincided with each other.

The resultant organic light emitting element was driven so that its luminance became 100 cd/m². As a result, its emission efficiency was 3.7 cd/A and red light emission was observed.

[Results and Discussion]

As described in Examples, when the electron injection layer is a mixed film containing the viologen compound X represented by the general formula [1] as an electron donor-property compound and the electron acceptor-property compound Y, an electron can be injected from the electrode (cathode) and hence light emission from the emission layer can be obtained.

As described above, the electron injection layer of the organic light emitting element of the present invention contains the electron donor-property viologen compound X (viologen compound represented by the general formula [1]) and the electron acceptor-property compound Y. Accordingly, the organic light emitting element of the present invention is an element stable against the air.

According to the present invention, the organic light emitting element having stable performance in the air can be provided.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2014-060339, filed Mar. 24, 2014 and Japanese Patent Application No. 2015-001212, filed Jan. 7, 2015, which are hereby incorporated by reference herein in their entirety. 

What is claimed is:
 1. An organic light emitting element, comprising: an anode; a cathode; and an emission layer placed between the anode and the cathode, wherein: the organic light emitting element further comprises a first organic compound layer placed between the cathode and the emission layer, and a second organic compound layer placed between the emission layer and the first organic compound layer, and contacted with the first organic compound layer; the first organic compound layer contains a first organic compound; the second organic compound layer contains a second organic compound; and the first organic compound comprises a viologen compound represented by the following general formula [1] and the second organic compound comprises an organic compound different from the viologen compound:

in the formula [1], R₁ to R₆ each represent a hydrogen atom or a substituent selected from the group consisting of a fluorine atom, an alkyl group, an alkoxy group, an aryl group, and a heteroaryl group, when any one of R₁ to R₆ represents an alkyl group or an alkoxy group, the alkyl group or the alkoxy group may further have a fluorine atom, and when any one of R₁ to R₆ represents an aryl group or a heteroaryl group, the aryl group or the heteroaryl group may further have a fluorine atom, an alkyl group, an alkoxy group, or an amino group.
 2. The organic light emitting element according to claim 1, wherein the first organic compound layer is contacted with the cathode.
 3. An organic light emitting element, comprising: an anode; a cathode; and an emission layer placed between the anode and the cathode, wherein: the organic light emitting element further comprises an organic compound layer placed between the cathode and the emission layer; the organic compound layer contains a first organic compound and a second organic compound; and the first organic compound comprises a viologen compound represented by the following general formula [1] and the second organic compound comprises an organic compound different from the viologen compound:

in the formula [1], R₁ to R₆ each represent a hydrogen atom or a substituent selected from the group consisting of a fluorine atom, an alkyl group, an alkoxy group, an aryl group, and a heteroaryl group, when any one of R₁ to R₆ represents an alkyl group or an alkoxy group, the alkyl group or the alkoxy group may further have a fluorine atom, and when any one of R₁ to R₆ represents an aryl group or a heteroaryl group, the aryl group or the heteroaryl group may further have a fluorine atom, an alkyl group, an alkoxy group, or an amino group.
 4. The organic light emitting element according to claim 3, wherein a content of the second organic compound is more than 0 wt % and 80 wt % or less with respect to a total of the viologen compound and the second organic compound.
 5. The organic light emitting element according to claim 1, wherein the viologen compound and the second organic compound satisfy the following formula [4]: |V _(red) −V _(ox)|≦1.0 V  [4] in the formula [4], V_(ox) represents a first oxidation potential value of the viologen compound and V_(red) represents a first reduction potential value of the second organic compound.
 6. The organic light emitting element according to claim 1, wherein the second organic compound is represented by any one of the following general formulae [2-1] to [2-21], [3-1], and [3-2]:

in the formulae [2-1] to [2-21], [3-1], and [3-2], R₂₁ to R₉₉ and R₁₀₃ to R₁₅₉ each represent a hydrogen atom or a substituent selected from the group consisting of an alkyl group, an alkoxy group, an aryl group, a heteroaryl group, and a fluorine atom, and when any one of R₂₁ to R₉₉ and R₁₀₃ to R₁₅₉ represents an aryl group or a heteroaryl group, the aryl group or the heteroaryl group may further have an alkyl group, an alkoxy group, an aryl group, a heteroaryl group, or a fluorine atom; in the formula [3-1], a unit Q represents a basic structure represented by any one of the formulae [2-1] to [2-21] or a partial structure including an aromatic ring having 6 to 30 carbon atoms, m represents an integer of from 0 to 6, and a unit P represents any one of the following substituents:

in the formulae, R₁₀₀ and R₁₀₁ each represent a substituent selected from the group consisting of a hydrogen atom, an alkyl group, an alkoxy group, an aryl group, a heteroaryl group, and a fluorine atom, and when any one of R₁₀₀ and R₁₀₁ represents an aryl group or a heteroaryl group, the aryl group or the heteroaryl group may further have an alkyl group, an alkoxy group, an aryl group, a heteroaryl group, or a fluorine atom, and * represents an atomic bonding with the unit Q; and in the formula [3-2], a unit T represents a partial structure including an aromatic ring having 6 to 30 carbon atoms or a partial structure including a five- or six-membered heterocyclic structure formed of a carbon atom and an oxygen atom, n represents an integer of from 0 to 6, and a unit Z represents any one of the following substituents:

in the formulae, R₁₀₂ represents a substituent selected from the group consisting of a hydrogen atom, an alkyl group, an alkoxy group, an aryl group, a heteroaryl group, and a fluorine atom, and when R₁₀₂ represents an aryl group or a heteroaryl group, the aryl group or the heteroaryl group may further have an alkyl group, an alkoxy group, an aryl group, a heteroaryl group, or a fluorine atom, and * represents an atomic bonding with the unit T.
 7. The organic light emitting element according to claim 1, wherein the viologen compound is represented by the following general formula [6]:

in the general formula [6], Ar₁ and Ar₂ each represent an aryl group that may have a fluorine atom, an alkyl group, an alkoxy group, or an amino group.
 8. A display apparatus, comprising a plurality of pixels, wherein at least one of the plurality of pixels includes the organic light emitting element of claim 1, and an active element connected to the organic light emitting element.
 9. A display apparatus, comprising a plurality of pixels, wherein at least one of the plurality of pixels includes the organic light emitting element of claim 3, and an active element connected to the organic light emitting element.
 10. The display apparatus according to claim 8, wherein the active element comprises a transistor and the transistor has an oxide semiconductor as an active layer.
 11. An image information processing apparatus, comprising: an input portion configured to input image information; and a display portion configured to display an image, wherein the display portion comprises the display apparatus of claim
 8. 12. A lighting apparatus, comprising: the organic light emitting element of claim 1; and an AC/DC converter configured to supply a driving voltage to the organic light emitting element.
 13. A lighting apparatus, comprising: the organic light emitting element of claim 3; and an AC/DC converter configured to supply a driving voltage to the organic light emitting element.
 14. The lighting apparatus according to claim 12, wherein: the organic light emitting element is connected to a transistor; and the transistor has an oxide semiconductor as an active layer.
 15. An image forming apparatus, comprising: a photosensitive member; a charging portion configured to charge the photosensitive member; an exposure portion configured to expose the photosensitive member; and a developing portion configured to apply a developer to the photosensitive member, wherein the exposure portion includes the organic light emitting element of claim
 1. 16. An image forming apparatus, comprising: a photosensitive member; a charging portion configured to charge a surface of the photosensitive member; an exposure portion configured to expose the photosensitive member; and a developing portion configured to apply a developer to the photosensitive member, wherein the exposure portion includes the organic light emitting element of claim
 3. 17. The image forming apparatus according to claim 15, wherein: the organic light emitting element is connected to a transistor; and the transistor has an oxide semiconductor as an active layer.
 18. An exposing apparatus, which is configured to expose a photosensitive member, the exposing apparatus comprising a plurality of emission points each including the organic light emitting element of claim 1, wherein the plurality of emission points are placed to form a line along a long axis direction of the photosensitive member.
 19. The exposing apparatus according to claim 18, wherein: the organic light emitting element is connected to a transistor; and the transistor has an oxide semiconductor as an active layer.
 20. An exposing apparatus, which is configured to expose a photosensitive member, the exposing apparatus comprising a plurality of emission points each including the organic light emitting element of claim 3, wherein the plurality of emission points are placed to form a line along a long axis direction of the photosensitive member. 